Apparatus for compensating an induction flow meter for fluctuations in the a.c. supply voltage

ABSTRACT

A circuit for compensating for the fluctuations of an a.c. line voltage on the measurement signal of a measurement pick-up element fed with the a.c. line voltage, in which an amplifier is connected in the output of the primary pick-up element whose amplification factor is changed inversely proportional to the influence of the a.c. line voltage fluctuations on the signal produced by the primary pick-up element wherein the amplifier is provided with a negative feedback circuit including a variable resistor which is controlled in response to a reference signal dependent on the AC supply voltage as well as by a constant auxiliary signal applied directly to the variable resistor so that the value of this resistor is controlled as a function of the difference between the auxiliary signal and the reference signal.

United States Patent [191 DiMarco 1 Oct. 9, 1973 APPARATUS FORCOMPENSATING AN 2,757,538 8/1956 Soffel 73/194 EM INDUCTION FLOW METERFOR 3,094,870 6/1963 Mittlemann 73/194 EM FLUCTUATIONS IN THE A.C.SUPPLY VOLTAGE Inventor: Franco P. DiMarco, Stuttgart-Bad Cannstatt,Germany J. C. Eckardt AG, Stuttgart-Bad Cannstatt, Germany Filed: Apr.22, 1971 Appl. No.: 136,400

Assignee:

Foreign Application Priority Data Apr. 23, 1970 Germany P 20 19 613.8

Ramp et al 324/34 FL 5 AMPLIFIER Primary ExaminerRobert J. CorcoranAttorney-Craig, Antonelli & Hill [5 7 ABSTRACT A circuit forcompensating for the fluctuations of an a.c. line voltage on themeasurement signal of a measurement pick-up element fed with the a.c.line voltage, in which an amplifier is connected in the output of theprimary pick-up element whose amplification factor is changed inverselyproportional to the influence of the a.c. line voltage fluctuations onthe signal produced by the primary pick-up element wherein the amplifieris provided with a negative feedback circuit including a variableresistor which is controlled in response to a reference signal dependenton the AC supply voltage as well as by a constant auxiliary signalapplied directly to the variable resistor so that the value of thisresistor is controlled as a function of the difference between theauxiliary signal and the reference signal.

2 Claims, 2 Drawing Figures RECT- l :'e'o'o'e L I 1nl amaze: 3 7 NEG.FEEDBACK NETWORK (CONTROL DEVICE PATENTED W 9 I975 5 AMPLIFIER NEG.FEEDBACK NETWORK /8 Fig.1 v

(CONTROL oavnce Ref.

RECT.

Fig. 2

INVENTOR FRANCO P. DI: MARCO APPARATUS FOR COMPENSATING AN INDUCTIONFLOW METER FOR FLUCTUATIONS IN THE A.C. SUPPLY VOLTAGE The presentinvention relates to a circuit for the compensation of the(multiplicative) influence of the fluctuations of the a.c. line voltage,especially of a magnetic-inductive flow meter which has an amplifierconnected in its output.

A compensation circuit is already known in the prior art in which withthe aid of a measuring bridge consisting of controllable resistancesthat are fed from a d.c.-and a.c.-voltage, the a.c. supply voltagechanges with the amplitude of the a.c. line voltage. However, theexpenditures as regards circuitry are very great in such prior artdevice.

The present invention is concerned with the task to avoid the circuitexpenditures, and to provide with few means an operationally reliablecompensation circuit that can be utilized generally for line-fed ormainspowered a.c. primary pick-up elements or a.c. measurementtransmitter elements, especially, however, for magnetic-inductive flowmeters with an amplifier connected in the output thereof.

According to the present invention, an installation is provided whichchanges the amplification factor inversely proportional to the influenceof the fluctuations of the a.c. line voltage on the measurement signalproduced by the primary pick-up or transmitter. Furthermore, provisionis made to form a reference voltage U from the current which feeds thetransmitter or primary pick-up element, and that this reference voltagewhich contains the line voltage fluctuations to the same extent as themeasurement voltage U M produced by the primary pick-up or transmitter,changes the amplification factor inversely proportionally to thefluctuatons of the a.c. line voltage. For that purpose, a controllableresistance which changes corresponding to the reference voltage, isarranged in the negative feedback circuit of the amplifier. These andfurther objects, features and advantages of the present invention willbecome more obvious from the following description when taken inconnection with the accompanying drawing which shows, for purposes ofillustration only, one embodiment in accordance with the presentinvention, and wherein:

FIG. 1 is a schematic block diagram indicating the principle of thepresent invention; and

FIG. 2 is a circuit diagram illustrating one embodiment for amagnetic-inductive flow meter in accordance with the present invention.

The measurement voltage U produced by the primary pick-up or transmitterelement 1 is proportional to the flow Q and to the induction B. If themeasurement voltage is amplified by the amplifier by a factor of V, thenthe following equation is true if the output voltage of the amplifier isdesignated by U The line voltage fluctuations express themselves incorresponding fluctuations of the magnetic induction B. This is to betaken into consideration by the factor A so that one can write theexpression for B as follows:

B=A B (3) whereby U is now a measure for the flow Q independently of thefluctuations of the a.c. line voltage. With other types of a.c. poweredmeasurement transmitter or primary pick-up elements, another physicalmagnitude exists in the place of the flow Q.

Referring now to the drawing, wherein like reference numerals are usedthroughout the two views to designate corresponding parts, and moreparticularly to FIG. 1, the magnets 2 of the magnetic-inductive flowmeter 1 of conventional construction are energized by the winding 3 feddirectly from the a.c. line voltage. In addition to an interferencevoltage of no interest, the measurement voltage U results at theelectrodes 4, which is fed to the input circuit of the amplifier 5. Anaverage value is formed by the rectifier 7 from the a.c. line voltage U,as reference voltage U This reference voltage U changes with thefluctuations of the a.c. line voltage to the same extent as themeasurement voltage U The reference voltage, however, could also bederived directly from the transmitter or pick-up magnets.

The reference voltage U acts by way of the control device 8 of any knownconstruction on the negative feedback network 9 of the amplifier 5 insuch a manner that with increasing reference voltage, the negativefeedback is increased and therewith the amplification of the amplifier 5decreases.

FIG. 2 illustrates one embodiment In accordance with the presentinvention.-

The measurement voltage U is fed to the transistor I amplifier 10 by wayof a capacitor. The resistances R, and R serve in a conventional manneras a voltage divider. The amplified signal can be picked up at thecollector by way of the average value rectifier 17 of conventionalconstruction. A network is connected in parallel with the emitterresistance R, which consists of the capacitor 11, the controllableresistance R 12 and and g the comparison resistance R 13. The comparisonresistance R,, is connected ground by way of the capacitor 14, insofaras the a.c. voltage is concerned If, in the network then the followingequation is true for the impedance Z of this arrangement:

If the capacitance of the capacitor 11 is selected sufficiently largeand, if R 'R, then The following then is true for the amplification V:

V R /R The constant auxiliary current I flows through the variablecontrollable resistance R 12, also called at times an electronicpotentiometer of known construction, and produces the auxiliary voltageU which is also constant.

The reference voltage U is formed by the rectifier 16 by appropriateaveraging either from the line voltage U,,, directly or from the pick-upmagnets 2. The auxiliary voltage U,,, is compared with the referencevoltage U in the differential amplifier 15. The differential amplifier,in case of deviations from a nominal value which result from thevariable magnitude of the reference voltage, controls the variablecontrollable resistance R 12 by conventional means in such a manner thatthe negative feedback of the amplifier is increased with increasingreference voltage and therewith the amplification factor changesinversely proportional. The output voltage U,, therefore will reflectonly the actual value of interest of the pick-up or transmitter element,independently of the ac. line voltage fluctua tions.

While I have shown and described only one embodiment in accordance withthe present invention, it is understood that the same is not limitedthereto, but is susceptible of numerous changes and modifications asknown to those skilled in the art, and I therefore do not wish to belimited to the details shown and described herein, but intend to coverall such changes and modifications as are encompassed by the scope ofthe appended claims.

I claim:

1. In an induction flow meter including an AC. supply voltage sourceconnected to field generating means and a primary pickup element, a mainamplifier connected to the output of said primary pickup element andhaving a negative feedback path which includes a variable resistor, andmeans for varying the resistance value of said resistor in response to areference voltage signal dependent on the AC. supply voltage, theimprovement consisting of a circuit arrangement for compensating for theeffect of fluctuations in the AC. supply voltage comprising means forfeeding a constant auxiliary current to the variable resistor to producean auxiliary voltage at this resistor, said means for varying saidvariable resistor including a differential amplifier to whose inputs arefed the auxiliary voltage and said reference voltage which isapproximately proportional to the amplitude of the A.C. supply voltageand whose output is applied in control of said variable resistor, sothat the value of the variable resistor is controlled as a function ofthe difference between the auxiliary voltage and said reference voltage.

2. Circuit arrangement according to claim 1, wherein said variableresistor is connected in series connection with a capacitor across anemitter resistor of a transistor serving as said main amplifier, whereinthe constant auxiliary current is isolated from the emitter circuit ofthe transistor by said capacitor.

1. In an induction flow meter including an A.C. supply voltage sourceconnected to field generating means and a primary pickup element, a mainamplifier connected to the output of said primary pickup element andhaving a negative feedback path which includes a variable resistor, andmeans for varying the resistance value of said resistor in response to areference voltage signal dependent on the A.C. supply voltage, theimprovement consisting of a circuit arrangement for compensating for theeffect of fluctuations in the A.C. supply voltage comprising means forfeeding a constant auxiliary current to the variable resistor to producean auxiliary voltage at this resistor, said means for varying saidvariable resistor including a differential amplifier to whose inputs arefed the auxiliary voltage and said reference voltage which isapproximately proportional to the amplitude of the A.C. supply voltageand whose output is applied in control of said variable resistor, sothat the value of the variable resistor is controlled as a function ofthe difference between the auxiliary voltage and said reference voltage.2. Circuit arrangement according to claim 1, wherein said variableresistor is connected in series connection with a capacitor across anemitter resistor of a transistor serving as said main amplifier, whereinthe constant auxiliary current is isolated from the emitter circuit ofthe transistor by said capacitor.